Seeing other mesh vertices while editing another one?
If you just need to display the edges, you can select the object, go in the Properties panel > Object > Display > Click on Draw All Edges and Maximum Draw Type > Wire.

But otherwise there is an addon called MultiEdit: https://www.dropbox.com/s/xtxbhxxw9qriti2/MultiEdit_1_0.zip?dl=0

Download and install it, then select both your objects and, in the left panel (activated with T) > Tools > Multiple Objects Editing > click on the MultiEdit Enter button. Click on the MultiEdit Exit when you have finished.